The invention relates to power conversion apparatus and methods, and more particularly, to uninterruptible power supply (UPS) apparatus and methods.
A typical conventional “on-line” UPS may include an AC/DC converter (e.g., a rectifier) that is configured to be coupled to an AC power source, such as a utility source, and a DC/AC converter (e.g., an inverter) that is coupled to the AC/DC converter by a DC link and which produces an AC voltage at an output (load) bus of the UPS. The UPS may further include a bypass circuit, e.g., a static switch, which can be used to couple the AC power source directly to the output bus of the UPS, such that the AC/DC converter and DC/AC converter are bypassed. The bypass circuit can be used, for example, to provide an economy mode of operation and/or to provide power to the load when either or both of the converters are damaged or inoperative.
Bypass circuits can create dangerous conditions in applications in which multiple UPSs are feeding a common load bus in parallel. In particular, a bypass source to a UPS in such a parallel-connected configuration may be absent due to, for example, tripping of a breaker or opening of a switch in the bypass source path. However, if the bypass circuit of the UPS is closed, voltage may be backfed from the common load bus through the closed bypass circuit. Accordingly, monitoring the voltage at an input of the bypass circuit may not reveal the absence of the bypass source, as the backfed voltage may provide an appearance that the bypass source is present. Underwriters Laboratories, Inc. (UL®) views such a state as a potential hazard, and has promulgated standards that require that such a condition be detected and avoided.